This invention relates generally to the field of accessing a communication medium. More particularly, certain embodiments of this invention relate to accessing a framed and slotted communication medium such as those complying with MPT1327, Tetra and similar protocols.
Wireless trunking communication systems utilize two or more communication channels to effect communication between two or more parties. One channel is utilized as a control channel which allocates the remaining communication channels to the users. The function of this control channel, for purposes of this document, is to set up calls between users.
In order to set up a call, a radio user initiates a call setup process by communicating with a central controller via the control channel with a request that a call be set up. The central controller then takes the remaining steps needed to set up the communication on one of the available communication channels.
The MPT1327 specification is published by the department of trade and industry (DTI) in the U.K. and it details a common signaling standard for land based trunked radio systems operating primarily in the U.K.""s VHF Band III Sub-band 1 and 2. Although this is a U.K. standard, this protocol has become a widely used standard for trunking systems across the world. The Tetra (Terrestrial Trunking Radio) standard is a newer standard that is growing in popularity throughout the world and may become the de-facto standard in the future. These standards, and other similar standards, define almost all of the communications activities of the communication system. Such activities include the signaling on the control channel that takes care of (among other things) the call set-up process. In order to set up a call, the control channel (a common channel) is accessed by the subscriber unit that is requesting a call. Since the calls in this type of system (and many others) are requested randomly, the method for accessing the control channel is often generically referred to as a Random Access Method. These access methods are often variations of the well known xe2x80x9cSlotted ALOHAxe2x80x9d Random Access Method.
Generally speaking, the central system controller communicates with the radios in the system over the control channel using a repeating frame of information. The frame contains control information as well as a series of contention slots. The contention slots are made available to the radio units for purposes of securing access to the control channel. According to most Random Access standards, there is defined a minimum and maximum number of slots in the frame. The equipment manufacturers have freedom to determine how many contention slots are in the frame at any given time subject to this minimum and maximum.
In order for a radio to place a call request message, it first has to select one of the contention slots on the control channel allocated for call request purposes. The contention slot accessed is generally selected randomly so that if several radios are attempting to secure a contention slot, their signals are unlikely to collide. Of course, as traffic load on the system increases, collisions become more frequent as the radios attempt to secure use of the same slot. This results in inefficient use of the control channel and delays in securing a call set up. This problem is generally addressed by increasing the number of contention slots per frame. This provides more contention slots from which each radio can randomly select, thus, reducing the probability of collision. As a channel becomes less loaded, fewer contention slots are needed to more efficiently use the channel capacity.
Several techniques have been devised to optimize the number of contention slots in systems such as this and thereby minimize channel access delay and make the most efficient use of the control channel""s bandwidth. Known techniques often require the radios to report statistics back to the central controller. The central controller accumulates and uses these statistics to make a determination of channel loading based on these statistics. Unfortunately, this reporting process uses control channel bandwidth, and thus, may contribute to inefficient use of the control channel. Delays can actually increase and traffic throughput can decline.
U.S. Pat. No. 4,398,289 to Schoute shows one example of a variation in the slotted ALOHA protocol which adjusts the frame length in accordance with the traffic on a channel. The technique used in this patent is simulated and compared with standard slotted ALOHA and the present invention at a later point in this document and is referred to as xe2x80x9cLoad Estimation.xe2x80x9d
There is need for a method for allocating the contention slots in such a system which can operate efficiently and preserve data throughput while minimizing delays. It is advantageous if such method is quickly adaptive, simple to implement and flexible.